Method Article

Morphology Control for Fully Printable Organic–Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

DOI:

10.3791/54923

⸱

January 10th, 2017

 ,  ,  , 
1Department of Mechanical Engineering, National Institute of Technology, Oyama College, 2Ashizawa Finetech Ltd.

In This Article

Summary

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A method for fully printable, fullerene-free, highly air-stable, bulk-heterojunction solar cells based on Ti alkoxides as the electron acceptor and the electron-donating polymer fabrication is described here. Moreover, a method for controlling the morphology of the photoactive layer through the molecular bulkiness of the Ti-alkoxide units is reported.

Abstract

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The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting material. However, fullerene derivatives are air-sensitive; therefore, air-stable material is needed as an alternative. In the present study, we propose and describe the properties of Ti-alkoxide as an alternative electron-accepting material to fullerene derivatives to create highly air-stable BHJ solar cells. It is well-known that controlling the morphology in the photoactive layer, which is constructed with fullerene derivatives as the electron acceptor, is important for obtaining a high overall efficiency through the solvent method. The conventional solvent method is useful for high-solubility materials, such as fullerene derivatives. However, for Ti-alkoxides, the conventional solvent method is insufficient, because they only dissolve in specific solvents. Here, we demonstrate a new approach to morphology control that uses the molecular bulkiness of Ti-alkoxides without the conventional solvent method. That is, this method is one approach to obtain highly efficient, air-stable, organic-inorganic bulk-heterojunction solar cells.

Introduction

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Organic photovoltaic devices are considered promising renewable energy sources due to their low manufacturing cost and light weight1-7. Because of these advantages, a large number of scientists have been immersed in this promising area. In the past decade, dye-sensitized, organic thin-film, and perovskite-sensitized solar cells have achieved significant progress in power conversion efficiency in this area8.

Specifically, organic thin-film solar cells and BHJ organic thin-film solar-cell technology are efficient and cost-effective solutions for the utilization of solar energy. Furthermore, the energy conversion efficiency has reached over 10% with the use of low-band-gap polymers as the electron donor and fullerene derivatives as the electron acceptor (Phenyl-C61-Butyric-Acid-Methyl Ester: [60]PCBM or Phenyl-C71-Butyric-Acid-Methyl Ester: [70]PCBM)9-11. Moreover, some researchers have already reported the importance of the BHJ structure in the photoactive layer, which is constructed with low-band-gap polymers and fullerene derivatives to obtain a high overall efficiency. However, fullerene derivatives are air-sensitive. Therefore, an air-stable electron-accepting material is required as an alternative. A few reports previously suggested new types of organic photovoltaic cells that used n-type semiconducting polymers or metal oxides as electron acceptors. These reports supported the development of air-stable, fullerene-free, organic thin-film solar cells12-15.

However, in contrast to fullerene systems or n-type semiconducting polymer systems, obtaining a satisfactory performance of the BHJ structure in the photoactive layer, which has charge separation and charge transfer abilities, is difficult in metal oxide systems16-17. Furthermore, fullerene derivatives and n-type semiconducting polymers have high solubility in many solvents. Therefore, it is easy to control the morphology of the photoactive layer by selecting an ink solution as the solvent, which is the precursor of the photoactive layer18-20. In contrast, in the case of metal alkoxide systems used in combination with an electron-donating polymer, both semiconductors are insoluble in almost all solvents. This is because metal alkoxides do not have a high solubility in the solvent. Therefore, the selectivity of solvents for morphology control is extremely low.

In this article, we report a method for controlling the morphology of the photoactive layer by using molecular bulkiness to fabricate printable and highly air-stable BHJ solar cells. We describe the importance of morphology control for the progress of fullerene-free BHJ solar cells.

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Protocol

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1. Preparation of Indium-tin-oxide (ITO) Glass for Solar Cell Fabrication

  1. Cut the ITO/glass substrate.
    1. Using a glass cutter, cut the ITO/glass substrate (10 cm × 10 cm) into pieces measuring approximately 2 cm × 2 cm.
  2. Chemically etch the ITO conductive layer.
    1. Using a digital multimeter, check that the top of the ITO/glass piece has a conductive side.
    2. Place masking tape on both sides of the ITO/glass piece, leaving a central area of 2 mm × 2 cm in the middle. Using masking tape, protect the rest of the ITO conductive layer from the etching.
    3. Pour a few drops of HCl (1 M) onto the ITO conductive layer to remove the ITO conductive layer from the surface of the ITO/glass piece. After approximately 3 min, wipe off the HCl using a cotton swab, and then remove the masking tape.
  3. Pretreat the ITO/glass piece.
    1. Place the ITO/glass pieces in a glass case and fill the case with water.
    2. Place the glass case in a water bath that is two-thirds full of water and attach an ultrasonic cleaner. Then, turn on the ultrasonic cleaner for approximately 15 min to remove the few traces of chemical etchant remaining on the ITO/glass piece. Wash these pieces in an ultrasonic bath with water, acetone, and isopropyl alcohol, respectively, for 15 min each, and then dry them in a stream of dry air. Perform ultrasonication at an oscillatory frequency of 42 kHz.
    3. Place the ITO/glass pieces inside an ultraviolet-ozone (UV-O3) cleaner and run the machine for 30 min.

2. Preparation of the Precursor Solution for the Photoactive Layer

  1. Dissolve 0.5 mg of poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) as an electron donor and 1.0 mg of Ti-alkoxide in 1 mL of chlorobenzene. Select the following Ti-alkoxides as electron acceptors: Ti(IV) isopropoxide, ethoxide, butoxide, and butoxide polymer. Then, dissolve 0.5 mg of PFO-DBT and 1.0 mg of [60]PCBM in 1 mL of chlorobenzene as a reference.
    NOTE: Here, the HOMO-LUMO levels are as follows21. PFO-DBT: 5.4-3.53 eV, Ti(IV) isopropoxide: 7.49-3.86 eV, ethoxide: 7.55-3.90 eV, butoxide: 7.53-3.76 eV, and butoxide polymer: 7.57-3.83 eV.
  2. On a magnetic hot stirrer, heat the precursor solution to 70 °C while stirring it with a stir bar at a rotational speed of 700 rpm. Do this for 20 min in the absence of light, until the solution is visually clear to the naked eye. Cool the solution to room temperature, again in the absence of light, for future use.

3. Fabrication of the Photoactive Layer

  1. Deposit the film by spin-coating.
    1. Heat the precursor solution of the photoactive layer and the ITO/glass piece to 70 °C. For 10 min, heat the precursor solution on a magnetic hot stirrer heated to 70 °C and use a stir bar at a rotational speed of 700 rpm. Heat the ITO/glass piece on a ceramic hot plate heated to 70 °C for 5 min.
    2. Place the ITO/glass piece at the center of the vacuum stage of the spin coater, heat it with a heat gun to around 70 °C, and turn on the vacuum.
      NOTE: The vacuum is created by using a vacuum pump with a pumping speed of 30 L/min. The ultimate pressure of the vacuum pump is 26.6 × 103 Pa.
    3. Pour a few drops of the precursor solution of the photoactive layer onto the ITO/glass piece and start the spin coater at 2,000-6,000 rpm for 60 s in air.
      NOTE: The volume of precursor solution is 0.5 mL, measured with a 1-mL spuit.
    4. Dry the surface of the photoactive layer for 10 min at room temperature in an air atmosphere in the absence of light to obtain a 50 nm-thick film as the photoactive layer.
  2. Remove the extra film.
    1. Wipe the extra photoactive layer from the surface of the ITO/glass piece with a cotton swab wetted with chlorobenzene.
    2. Dry the photoactive layer again for 10 min at room temperature in an air atmosphere in the absence of light.
      NOTE: The temperature of our experiment room is maintained at 25 °C.

4. Fabrication of the Electrode

  1. Print the organic electrode.
    1. Using a screen printer, print an organic electrode by placing poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) on the photoactive layer22. The metal mask is 50 µm-thick, and the printing area is 5 mm × 20 mm.
    2. Dry the organic electrode for 30 min at room temperature in an air atmosphere in the absence of light.

5. Lamination for the Solar Cells

  1. Cut the glass substrate into pieces with dimensions of 1.5 cm × 2.5 cm using a diamond cutter. Spread an epoxy resin onto the glass substrate using a plastic spatula. Place the glass substrate with epoxy resin on the photoactive layer to protect it.

6. Preparation for Measuring the Solar-cell Performance

  1. Clean the electrodes by wiping them with a cotton swab wetted with acetone. Attach a supporting electrode onto the ITO using an ultrasonic soldering system. Operate the soldering iron at a 42-kHz frequency and at 230 °C.

7. Measurement of the Solar-cell Performance

  1. Measure the current–voltage (J-V) characteristics of the solar cells by using a direct-current voltage current source/monitor integrated system, with the solar simulator calibrated to provide a simulated AM1.5G of 100 mW/cm2 by the silicon photodiode.
    NOTE: More detailed information about the measurement of the J-V curves can be found elsewhere23, 24.

8. Analysis of the Phase-separation structure

  1. Prepare individual films of photoactive layers constructed with Ti-alkoxide and PFO-DBT, using same method for solar-cell fabrication, without the organic electrode and without the lamination process.
  2. Use an optical microscope or a scanning electron microscope (SEM) to observe the morphology of the photoactive layer at a high magnification (50,000×) in order to analyze the phase-separation structure.
    NOTE: More detailed information about SEM operation can be found elsewhere25, 26.

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Results

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We have presented a protocol for fabricating fully printable organic-inorganic BHJ solar cells, as well as a method for controlling the phase-separation structure. The solar-cell performance has been extensively investigated27-31 when Ti(IV) isopropoxide and ethoxide were used as electron-accepting materials (Figure 1). These solar cells exhibited a short-circuit current density (Jsc) that is approximately eight times higher than that of devices using the "Ti(I...

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Discussion

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In order to utilize the molecule's bulkiness in this method, it is important to know the conditions for film formation by spin coating. First, the p-type and n-type semiconductors must be able to be dissolved in the solvents. When some material remains, it will become the large core of the domains in the photoactive layer. The use of an adequate commercial filter for individual solvents is recommended to remove the remaining material. Next, the precursor solution in which the molecules dissolve must be uniformly and ...

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Disclosures

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The authors declare that they have no competing financial interests.

Acknowledgements

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This work was partially supported by JSPS KAKENHI Grant Number 25871029, the Nippon Sheet Glass Foundation for Materials Science and Engineering, and the Tochigi Industrial Promotion Center. The National Institute of Technology, Oyama College, also assisted with the publication costs of this article.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Ti(IV) isopropoxide, 97%Sigma Aldrich205273
Ti(IV) ethoxideSigma Aldrich244759Technical grade
Ti(IV) butoxide, 97%Sigma Aldrich244112Reagent grade
Ti(IV) butoxide polymerSigma Aldrich510718
Poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT)Sigma Aldrich754013
[6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) 99.5%Sigma Aldrich684449Research grade
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS)HeraeusClevios S V3
1 N Hydrochloric acidWako083-01095
Chlorobenzene 99.0%Wako032-07986
Acetone 99.5%Wako016-00346
Indium-tin oxide (ITO)-coated glass substrateGeomatec0002100×100×1.1t (mm)
Glass substrateMatsunami GlassS721376×26×1.2t (mm)
Cotton tail As one1-8584-16
Epoxy resinNichibanAR-R30
Plastic spatulaAs one2-3956-02
Ultrasonic cleanerAs oneAS482
Magnetic hot  stirrerAs oneRHS-1DN
Ceramic hotplateAs oneCHP-17DN
Spin coaterKyowarikenK-359 S1
Vacuum pumpULVACDA-30S
UV-O3 cleanerFilgenUV253E
Screen printerMitani ElectronicsMEC-2400
Ultrasonic Soldering systemKuroda TechnoSUNBONDER USM-5
Direct-current voltage and current source/monitor integrated systemSan-Ei ElectricXES-40S1
Scanning electron microscopeJEOL Ltd.JSM-7800

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Titanium Alkoxide Electron AcceptorsMorphology Control MethodPhase Separation StructureOrganic Inorganic HybridSpin Coating TechniqueScanning Electron MicroscopyCurrent Voltage CharacteristicsPFO DBT Donor SystemFully Printable Solar Cells

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